John tells us that he was in the Spirit in Revelation 1:10. Being in the Spirit does not refer to John having the Holy Spirit. The text literally says “I came to be in the spirit,” meaning he “came to be” in a trance (cp. Acts 10:10; 22:17). It is not the normal spiritual condition of a believer who is characterized as being in the Spirit instead of being in the flesh (Romans 8:9). Rather, it is an extraordinary condition, whereby a believer is able to witness spiritual phenomena that he could not ordinarily see and hear.
John said he came to be in the spirit on the Lord’s day or on the Day of the Lord. Folks have tried to show that, because Lord (G2960) is an adjective rather than a noun, then it couldn’t possibly be the long awaited Day of the Lord, and rather refers to ‘Sunday’ as the believer’s day of worship. Nevertheless, this understanding is purely subjective. There is absolutely no objective proof that we should understand John’s statement to refer to the first day of the week. On the contrary, there is evidence within the Bible that this should not be done. For example, is there any difference between the Lord’s Law (Exodus 13:9) and the Law of the Lord (2Kings 10:31; 1Chronicles 16:40; 22:12; Nehemiah 9:3; Luke 2:23)? Is there any difference between the Lord’s Passover (Exodus 12:11, 27; Leviticus 23:5) and the Passover of the Lord (Numbers 28:16)?
Although I disagree with the final conclusion of the authors of a thesis referred to below, I do find this statement very interesting concerning the Greek word for Lord (G2960) being used adjectively:
“The adjective kyriakos first appears in Greek literature in 1 Corinthians 11:20 and elsewhere in the NT only in Revelation 1:10. Nor is it used in non-Christian Jewish literature. In secular Greek, where the word begins to appear A.D. 68, after but independent of Paul’s usage, it almost always refers to the emperor and has the sense of “belonging to the emperor” or “imperial.”
“… Examination of usage leads to the conclusion that “in meaning the word kyriakos is simply synonymous with (tou) kyriou in all cases where (tou) kyriou is used adjectivally with a noun, with the exception of the objective genitive” (Bauckham, 224); kyriakos and (tou) kyriou seem to be interchangeable. The sparse usage of the adjective in the NT (twice) corresponds to contemporary usage in general and is probably not due to any special meaning.”
While ultimately the authors try to make a case for Sunday being called “the Lord’s day” they fail to do so, in that each and every example, which they use from the New Testament to point to Sunday, actually refers to a specific day of the year, not a specific day of the week (Matthew 28:1; Mark 16:2, 9; Luke 24:1; John 20:1, 19; Acts 20:7; 1 Corinthians 16:2). The first day of the week in each and every case of these scriptures is not only in the plural (first day of the weeks), but each example refers to that specific day of the year, which fell between the two annual Sabbaths that occur during the Days of Unleavened Bread (the Passover season).
Therefore, the translation: “the Lord’s day” (Revelation 1:10) cannot possibly be evidence to prove Sunday worship. I’m not saying we should all begin worshiping on Saturday, but I am saying Revelation 1:10 (and any of the above scriptures for that matter) cannot be used to refer to a Sunday worship service. The term the Lord’s Day is the same as the Day of the Lord, and John, therefore, was in the Spirit on **this** day. In other words the whole of the Apocalypse is a description of the long awaited Day of the Lord. This needs to be kept in mind as we read and study the rest of this book.
 Of the 40 translations of the New Testament that I have, only eight (EWB CB; ECB; HRB; ISV; JUB; TLV; TS 2009; Weymouth) translate Revelation 1:10 as: “(I was in the Spirit) on the Day of the Lord…” For all others it is: “…the Lord’s day.”
 Ralph P. Martin and Peter H. Davids, eds., Dictionary of the Later New Testament and Its Developments (Downers Grove, IL: InterVarsity Press, 1997).